The present invention relates to a transparent conductive film and a process for producing the film.
Transparent conductive films are used as an electrode for driving liquid crystal serving as a display element for an information terminal in a computer, a portable telephone or the like. Indium tin oxide is mostly used as the raw material for such film. However, there is a demand for production of transparent conductive film of lower resistance to realize a more high-speed and finer display element.
Reportedly the lowest resistivity of transparent conductive films ever produced is 1xc3x9710xe2x88x924 xcexa9cm. In other words, the prior art technologies were unable to produce a transparent conductive film having a resistivity as low as 5xc3x9710xe2x88x925 xcexa9cm which is the desired resistivity of future display elements.
A principal object of the present invention is to provide a transparent conductive film having a lower resistivity than conventional transparent conductive films.
In view of the foregoing level of prior art technologies, the present inventors conducted extensive research and found that a film composed of indium tin oxide containing a small amount of nitrogen shows a low resistivity and can be used as an excellent transparent conductive film. Based on this novel finding, the inventors completed the present invention.
The present invention provides the following transparent conductive films and processes for producing the same.
1. A transparent conductive film of 5 nm to 100 xcexcM in thickness comprising nitrogen-containing indium tin oxide formed on a substrate.
2. The transparent conductive film as defined in item 1 which has a nitrogen content of 0.01 to 10 atom %.
3. The transparent conductive film as defined in item 2 which has a nitrogen content of 1 to 5 atom %.
4. A process for producing a transparent conductive film, the process comprising the step of depositing vaporized indium tin oxide on the surface of a substrate which is led to a state of being excited in a vacuum.
5. The process as defined in item 4, wherein the surface of the substrate is excited by irradiation with an ion beam.
6. The process as defined in item 5, wherein the surface of the substrate is excited by irradiation with a nitrogen-containing oxygen ion beam.
7. The process as defined in item 6, wherein the irradiation is conducted with a nitrogen-containing oxygen ion beam, the nitrogen content being 0.1 to 30 atom %.
8. The process as defined in item 5, wherein the irradiation is conducted with a nitrogen-containing oxygen cluster ion beam, the cluster containing 0.1 to 30 atom % of nitrogen.
9. The process as defined in item 4, wherein the vapor deposition of indium tin oxide is performed by any of vacuum deposition method, laser abrasion method, ion plating method, ion beam deposition method and CVD method.
10. The process as defined in item 9, wherein the vapor deposition of indium tin oxide is performed using a sintered product of indium oxide and tin oxide.
11. The process as defined in item 9, wherein the vapor deposition of indium tin oxide is performed using metal indium and metal tin.
The statement xe2x80x9cthe surface of a substrate is led to a state of being excitedxe2x80x9d used herein refers to the state of substrate surface in which after irradiation with an ion beam of 0.1 or more mW/m2 in the density of energy flowing into the substrate surface to be irradiated, the substrate surface is given greater thermal motion energy or greater internal energy than before irradiation.
In the present invention, there is no limitation on materials for the substrate on which the transparent conductive film is formed. Useful materials include, for example, silicon wafers, glasses, ceramics, organic polymers and the like.
Useful glasses include, for example, oxide glasses, and preferred glasses include silicate glass, borosilcate glass, alkali glass, etc.
Useful ceramics include, for example, crystalline oxide ceramics, and preferred ceramics include alumina, magnesia zirconia, etc.
Useful organic polymers include, for example, polyimide, polyethylene, polyvinyl, etc., and preferred organic polymers include polymethyl methacrylate, polyimide, etc.
Among these materials, borosilicate glass is more preferred.
The transparent conductive film formed on the substrate according to the invention is composed of nitrogen-containing indium tin oxide. The nitrogen content is usually about 0.01 to about 10 atom %, preferably about 1 to about 5 atom %. The transparent conductive film usually has a thickness of about 5 nm to about 100 um, preferably about 10 nm to about 1 xcexcm. When the film is too thin, the film used as an electrode shows a high electrical resistance and is unsuitable for use. On the other hand, if the film is too thick, the film absorbs light to a more extent and is insufficient in transparency.
The transparent conductive film of the invention may be produced as explained below.
First, the surface of the substrate for forming the film is led to a state of being excited. To excite the substrate surface, the surface may be irradiated with an ion beam in the conventional manner. The irradiation conditions can be suitably selected according to the kind of substrate, desired film thickness and the like.
For excitation of substrate surface, use can be made of at least one of inert gases such as helium, neon, argon and krypton or at least one of ions of carbon, nitrogen, oxygen and nitrogen oxide. When an oxygen ion, a nitrogen ion or a nitrogen oxide ion is not used as the ion forming a beam, it is preferred to supply oxygen, nitrogen or nitrogen oxide over the substrate surface at the site of reaction to form the atmosphere. More preferably the ions of ion beam are not single-atom ions but multi-atom ions or cluster ions. The accelerating voltage of ion beam is not limited but is usually about 10 eV higher.
After excitation of substrate surface, vaporized indium and tin are deposited on the substrate surface excited in the above-mentioned manner to form a transparent conductive film. The vapor deposition can be conducted concurrently with the excitation of substrate surface.
The vapor deposition of indium and tin can be carried out by conventional film-forming methods such as resistance heating method, electron beam method, laser abrasion method, ion plating method, ion beam deposition method and CVD method. In the present invention, the formation of film can be accelerated by introducing oxygen, nitrogen, nitrogen oxide or the like into the vacuum in the film-forming operation.
Nitrogen can be introduced into indium tin oxide in the present invention by incorporation of nitrogen in at least one of the stage of exciting the substrate surface and the stage of vapor deposition.
According to the present invention, a transparent conductive film having a resistivity as low as 5xc3x9710xe2x88x925 xcexa9cm or less can be obtained.
Such transparent conductive film is very useful, for example, as an electrode for driving liquid crystal widely used as a display element.